The present invention relates to an optical pickup device.
There has been known an optical disc (two-layer disc) having two layers of information recording surfaces (recording layers), as a method, for example, to increase recording capacity by enhancing recording density of DVD (digital video disc).
The two-layer disc is of the structure wherein a transparent protective base board, a first information recording surface, an intermittent layer, a second information recording surface and a protective base board on the reverse side are superposed in this order from the light source in the direction of an optical axis.
In the two-layer disc, a distance (thickness) from the surface of the transparent protective base board to the second recording layer is thicker than that from the surface of the transparent protective base board to the first recording layer by an amount of the intermittent layer, owing to the aforementioned structure. Therefore, spherical aberration caused by this thickness difference arises on each information recording surface.
However, in the case of DVD wherein a numerical aperture of an objective lens closer to an image is as relatively small as 0.6, it is possible to conduct recording and reproducing of information without correcting spherical aberration, because the spherical aberration stated above is in a range where there are no difficulties in practical use.
Incidentally, in recent years, there have been advanced research and development of the so-called high density optical disc wherein recording density has been enhanced by employing a blue laser beam having a wavelength of about 400 nm, by making a numerical aperture (NA) of an objective lens closer to an image to be about 0.85 and by making a protective base board thickness of an optical disc to be 0.1 mm, and further, there has been advanced development of a technology to make such high density optical disc to be of the two-layer structure.
Then, in the high density optical disc having the two-layer structure, it is required to correct spherical aberration caused by a difference between the thickness from the surface of a transparent protective base board to a first information recording layer and the thickness from the surface of a transparent protective base board to a second information recording layer.